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J. Am. Chem. SOC.1992,114, 9943-9952 9943 Swedish Board for Technical Development (NUTEK) and Medivir (EMBO) through a 2-year EMBO fellowship to L.H.K. is AB, Lunastigen 7, S-141 44 Huddinge, Sweden, for generous gratefully acknowledged. financial support (to J.C.). Thanks are due to the Wallen- bergstiftelsen, ForskningsrAdsnPmnden, and University of Uppla Supplementary Material Available: Experimental details and for funds for the purchase of a 500-MHz Bruker AMX NMR tables of atomic coordinates, positional atomic coordinates, thermal spectrometer in J.C.'s lab. The Science and Engineering Research parameters, bond distances, bond angles, torsion angles, inter- Council and the Wellcome Foundation Ltd. are gratefully ac- molecular contacts, least-squares planes, and intensity data (24 knowledged for generous financial support (to R.T.W.). Financial pages); listing of structure factors (8 pages). Ordering information Support from the European Molecular Biology Organization is given on any current masthead page.
Self-Assembled Monolayers of Parent and Derivatized [ n]Staffane-3,3("-' )-dithiols on Polycrystalline Gold Electrodes
Yaw S. Obeng,+Mark E. Laing,' Andrienne C. Friedli, Huey C. Yang, Dongni Wang,f Erik W. Thulstrup,"Allen J. Bard,* and Josef Michl**l Contribution from the Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 7871 2-1 167. Received April 2, I992
Abstract: Synthesis of the terminally disubstituted rigid rod molecules, [n]staffane-3,3("')-dithiols, n = 1-4, and their singly functionalized derivatives carrying an acetyl or a pentaammineruthenium(II), [Ru(NH~)~]~+,substituent is described. Self-assembled monolayers of the neat [nlstaffane derivatives and of their mixtures with n-alkyl thiols were prepared on polycrystalline gold electrodes. Grazing incidence FTIR spectroscopy and comparison with polarized IR spectra of model compounds oriented in stretched polyethylene revealed that, in the films assembled directly from the dithiols, the average orientation of the [nlstaffane rods was halfway between perpendicular and parallel to the surface, whereas, in the films obtained from the singly functionalized derivatives, the rods were nearly perpendicular to the surface. The degree of ordering and packing was also investigated by contact angle measurements with deionized water. The electrode blocking properties of the films were investigated by three independent electrochemical techniques, cyclic voltammetry in a solution containing K,Fe(CN),, chronoamperometry in a solution of RU(NH,)~CI~,and cyclic voltammetry of the gold surface in sulfuric acid. The results from the three methods were in good agreement and showed that the films assembled from the dithiols were not as well organized as those obtained from the monoderivatized analogues. In situ hydrolytic removal of the protective acetyl groups and subsequent functionalization of the resultant film covered with free thiol groups with aquapentaammineruthenium(II), [RU(NH~)~(H~O)]*+, are possible without disturbing the structure of the film. Electrodes covered with monolayers carrying ruthenium pentaammine groups on their outer Surface exhibit oxidation-reduction waves at 0.51 V vs SCE attributable to the Ru(I1)-Ru(II1) couple, demonstrating electron transfer across the monolayer. The formal potential of the immobilized couple is about 60" more positive than that of the dissolved complex, [ [(NH3),Ru]-[n]staffanedithiol]2+,in 1 M HClO, solution. The surface concentration of Ru is the same in the films assembled from this complex and those produced by in situ functionalization of a film covered with free thiol groups and corresponds to a monolayer of (NH3)SRu. However, in the former type of fdm, the surface concentration of the staffane rods is only about half of that in the latter, implying that only about half of the surface thiol groups have been metalated.
Introduction (n = 1-4), as well as mixed films formed with n-alkanethiols. We Terminally disubstituted [nlstaffanes ([a] 1) were originally find that careful basic hydrolysis converts films of [a15 undamaged developed for use as assembly elements in a molecular-size "Tinkertoy" construction set14 but can also be viewed simply as (1) (a) Kaszynski, P.; Michl, J. J. Am. Chem. SOC.1988, 220,5225. (b) a new class of straight and fairly rigid inert spacers suitable for Kaszynski, P.; Friedli, A. C.; Michl, J. Mol. Cvst. Liq. Cryst. Lett. Sect. 1988, investigations of electron and energy transfer. In this regard, 6, 27. (2) Michl, J.; Kaszynski, P.; Friedli, A. C.; Murthy, G. S.;Yang, HA.; terminal disubstitution with thiol-based gro~ps~-~is particularly Robinson, R. E.; McMurdie, N. D.; Kim, T. In Strain and Its Implicotions interesting, and preliminary results of an optical investigation of in Organic Chemistry; de Meijere, A., Blechert, S., Ms.;NATO AS1 Series; intramolecular electron transfer between Ru(I1) and Ru(II1) Kluwer Academic Publishers: Dordrecht, The Netherlands, 1989; Vol. 273, attached to ([a]3, n = p 463. 3,3("')-bis(methylthio)[n]staffanes 1,2), (3) Friedli, A. C.; Kaszynski, P.; Michl, J. Tetrahedron Lett. 1989,30,455. generated in situ from the isovalent species [a]2, have been (4) Kaszynski, P.; Friedli, A. C.; Mich, J. J. Am. Chem. Soc. 1992, 214, presented elsewhere., In this paper, we describe the formation 601. of self-assembled monolayers of neat [n]staffane-3,3("')-dithiols (5) (a) Bunz, V.; Polborn, K.; Wagner, H.-V.; Szeimes, G. Chem. Ber. ([a]4), thiol-thiol acetates (monothiols, [n]5), and thiol-ruthe- 1988,221, 1785. (b) Wiberg, H. B.; Waddell, S. T. J. Am. Chem. Soc. 1990, 112, 2194. nium pentaammine thiols ([a]6)(Figure 1) on gold electrodes (6) (a) Michl, J.; Friedli, A. C.; Obeng, Y.S.; Bard, A. J. Proceedings o/ the Sixth International Conference on Energy and Electron Tronsfec Fiala, *To whom correspondence should be addressed. J.; Pokorny, J., Eds.; Charles University: Prague, Czechoslovakia, 1989. The 'Present address: AT&T Bell Labs, Allentown, PA 18103. rates reported in this paper are too high due to the use of an erroneous *Present address: Unilever Research, Port Sunlight Laboratory, Quarry literature equation for data treatment. The correct rate constants are 1 X lo7 Road East, Bebington, Wirral L63 3JW, England. and 2 X lo6 s-I for [1]3 and [2]3, respectively, in 0.1 M DCI in D20. (b) I Present address: Chemistry Department, The Royal Veterinary and Positions on the individual bicyclo[l.l.I]pentane cages in an [nlstaffane are Agricultural University, DK- 187 1 Frederiksberg, Denmark. distinguished by the number of primes, from no primes at one of the terminal Present address: Institute I, Roskilde University, DK-4000 Roskilde, cages to n - 1 primes on the other terminal cage. Thus, the terminal Denmark. bridgehead positions in (2lstaffane are 3 and 3', in [3]staffane, 3 and 3", and Resent address: Department of Chemistry and Biochemistry, University in [nlstaffane, 3 and 3 with n - 1 primes, indicated by 3(*'), following the of Colorado, Boulder, CO 80309-0215. usage common in mathematics.
0002-7863/92/15 14-9943%03.00/0 0 1992 American Chemical Society 9944 J. Am. Chem. SOC.,Vot. 114, No. 25, I992 Obeng et at. Absolute ethanol (Aaper Alcohol and Chemical Co., Shelbyville, KY), acetonitrile (Fisher Scientific), and acetone (Mallinkrodt, Paris, KY) were used as received. Equipment and Measurements. Melting points were determined using a Boetius PHMKOS apparatus, with microscope attachment, at a warm- up rate of 4 "C min ' or in a sealed capillary and are uncorrected. Boiling points are also uncorrected. NMR spectra were run at 360 MHz in CDCI, on a Nicolet NT-360 instrument, unless specified otherwise. Chemical ionization (CI) and electron impact (EI) mass spectra were obtained on Bell & Howell 21-491 and VG ZAB-2E spectrometers, respectively. Elemental analyses were performed at Atlantic Microlabs, Norcross, GA. Preparative gas chromatography was performed on 6-ft SE-30 20% on Chromosorb column. Infrared (IR) spectra of 3,3("')-bis(a~etylthio)[n]staffanes~*~([n]7, n = 1-4). aligned in stretched polyethylene, were recorded at room temperature in the 40+2000-cm-' region on a Perkin-Elmer 1702 FTIR instrument equipped with a SPECAC Model KRS-5 linear polarizer in the sample beam. The resulting spectra, E,(;), with the electric vector parallel to the polymer-stretching direction, and Ed;), with the electric vector perpendicular to this direction, were base-line corrected. Narrow regions near 730 and 1480 cm-' were blocked by the absorption of the polymer. Figure 1. Schematic representation of the chemical transformations of The polymer sheets were prepared by cutting a 2-mm thick wall of a a film of type A into a film of type E. polyethylene laboratory bottle (Kartell, Germany), heating it to 150 "C and quenching it in cold water to improve its optical quality, and then stretching it to 400% of its original length. The stretched sheet was kept into films of [n]4 and characterize the structures and the elec- in a concentrated solution of the staffane derivative in chloroform at 40 trochemical properties of all of these. The formation of a self- "C for up to 24 h, until a desired optical density of the solute was achieved. This had no significant effect on the degree of alignment of the polymer. All other infrared spectra were recorded at 4-cm-' resolution on a Nicolet 60SXR FTIR instrument using a HgCdTe detector cooled with liquid nitrogen. Isotropic spectra were measured in KBr pellets unless stated otherwise. Grazing incidence measurements on films deposited X Y film type on a 1 X I-in. Au-coated glass were performed at an incidence angle of H H 84". using a Harrick model RMA-OOG reflection attachment and a wire [ (NH,),RuMeS] 2A [ (NH,),RuMeS] 2+ grid polarizer (IGP225, Cambridge Physical Science). Typically 4000 [ (N H,),RuMeS] 2+ [ (N H,),RuMeS] 3+ scans were recorded. HS HS A, C Cyclic voltammetry measurements were performed with a PAR sys- HS CHJCOS B tem (model 175 universal programmer and 173 potentiostat) and a HS [(NH,),RuSl 2+ D, E, F Houston Instruments 200 X-Y recorder. In chronoamperometric ex- C H ,COS CHJCOS periments, current transients were stored on a Norland 3001 digital oscilloscope or were obtained with a BAS 100-B electrochemical analyzer assembled monolayer relies upon direct adsorption onto substrates interfaced to an IBM-XT personal contputer. Transient data were re- from solution and differs from the well-known Langmuir-Blodgett corded at several different sampling rates for each modified electrode to technique of monolayer preparation where highly ordered mon- obtain an accurate digitized current over the entire time window.15 olayers or systems of monolayers on solid supports are produced Either a saturated calomel electrode (SCE) or a silver wire quasi-refer- by mechanical transfer of preformed films from a liquidair in- ence electrode (QRE) was used as the reference electrode, and platinum terface. Monolamellar self-assembled films have been used as gauze served as the auxiliary electrode in all of the electrochemical interfaces for a wide variety of studies, e.g., in corrosion inhibition,' experiments. All electrochemical measurements were made at room temperature (24 i 2 "C). tribology,g and cataly~is.~Polycrystalline gold substrates were Contact angle measurements were made with a Rami?-Hart Model used as electrodes because the well-known strong specific inter- 100 goniometer equipped with a tilting stage. A sessile 10-pL drop of action of gold with sulfur facilitates self-a~sembly.~O-~~ Milli Q water was dropped from a microsyringe onto the sample and the contact angle measured. The stage was then inclined to 45O, and the Experimental Section receding and advancing contact angles were measured. The reported Materials. 18 MR-cm Milli-Q water (Milli Q,El Paso, TX) was used values are the average of at least three different measurements in every throughout this study for washings, solution preparation, and contact case. angle measurements. The electrolytes used were all reagent grade. General Synthetic Prdure for Monothiols ([n15). 3,3"'-')-Bis(ace- tylthio)[n]staffane ([n]7)3.4(in most cases 1 mmol) was dissolved in ~~ degassed ethanol (10 mL/1 mmol) under argon. One equivalent of KOH (7) Bregman, J. I. Corrosion Inhibitors; MacMillan: New York, 1963; in ethanol was added dropwise. For n = 3 and n = 4 oligomers, the Chapter 5, p 197. reaction was carried out in gently refluxing ethanol and in 2-propanoI. (8) (a) Bowden, F. B.; Tabor, D. The Friction and Lubricurionofsolids; Oxford University Press: London, 1968; Part 11, Chapter 19. (b) Zisman, respectively, to improve solubility. The solvent was then evaporated, and N. A. In Friction and Wear, Davies, R., Ed.; Elsevier: New York, 1959. equal volumes of water and methylene chloride were added. Acidification (9) (a) Murray, R. W. In Elecrroanalytical Chemistry; Bard, A. J., Ed.; with acetic acid under argon gave milky layers, which were separated, Marcel Dekker: New York, 1984; Vol. 13. (b) Durrand, R. R.; Bencosme, and the aqueous layer was extracted three times with methylene chloride. C. S.; Collman, J. P.; Anson, F. C. J. Am. Chem. Soc. 1983, 105, 2710. The combined organic layers were dried with Na2S04and evaporated (IO) Tillman, N.; Ulman, A.; Elman, J. F. Langmuir 1989, 5. 1020. to give dithiol, monothiol, and starting material in statistical proportions (11) Nuzzo. R. G.; Allara, J. L. J. Am. Chem. Soc. 1983. 105, 4481. (1:2:1, respectively, except for the case of n = 1, where monothiol pre- (12) (a) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall. J.; Whitesides. dominated) by GC analysis. After evaporation of solvent, the products G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, Ill, 321. (b) Whitesides. G. M.; Laibinis, P. E. Longmuir 1990.6.87. (c)Strong, L.; Whitesides, G. M. were separated and purified by column chromatography on silica gel (3% Langmuir 1W,4,546. (d) Ogletree, D. F.; Ocal, C.; Marchon, B.; Somorjai, ethyl acetate in hexanes eluent) and then sublimation. Preparative GC G. A.; Salmeron. M.; Beebe, T.; Siekhaus. W. J. Vac. Sci. Technol., A. 1990. could also be used when n < 3, but significant decomposition resulted 8. 297. with higher oligomers. (13) Facci, J. S.; Falcigno, P. A.; Gold, J. M. Longmuir 1986, 2, 732. Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. ( 15) (a) Gueshi, T.;Tokuda, K.; Matsuda, H. J. Elecrrwnal. Chem. 1978, (14) (a) Finklea, H. 0.;Avery, S. Lynch, M. Lonpuir 1987.3.409. (b) 89,247. (b) Tokuda, K.; Gueshi, T.; Matsuda, H. J. Elecrrwnal. Chem. 1979, Moat. R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100.456. (c) Finklea. H. 101.29. (c) Tokuda, K.; Gueshi, T.; Matsuda, H. J. Electrwnal. Chem. 1W9, 0.;Hanshew. D. D. J. Am. Chem. Soc. 1992, 114. 3173. 102, 41. (d) Leddy, J.; Bard, A. J. J. Electround. Chem. 1983, 153, 223. [n]Staffone- 3,3(”’)-Dithiols J. Am. Chem. SOC.,Vol. 114, No. 25, 1992 9945
General Synthetic Procedure for Dithiols ([ny).When only the dithiol substrate was cleaned prior to deposition in a fresh mixture of 1:3 v/v was required, the above procedure was followed using 2 equiv or excess H202(30%)/H2S04 (96%) for 30 min, rinsed with water followed by of base. Purification was simplified greatly to Kugelrohr distillation (n ethanol, dried in an oven at 160 OC, and then subjected to 2 min of argon = I), flash column chromatography on silica gel with 10% ethyl acetate plasma etching prior to sputtering.17 The glass was primed with a in hexanes eluent, or sublimation (n > 1). Yields are reported for dithiols -50-A Cr layer followed by a -20004 Au layer. This afforded Au prepared with this method. disk electrodes with a mean geometric area of 0.18 cm2. Electrical Bicyclo(l.l.llpentane-1,3-dithiol ([1]4).* Volatile liquid with a strong contact was made to the working electrode via an alligator clip at a larger stench; yield 78%; bp 170-180 OC (Kugelrohr); ‘H NMR 6 2.1 1 (s, 2 H), rectangular gold patch which was -2 cm from the disk. The electrodes 2.18 (s, 6 H); ”C NMR 6 34.97, 62.19; IR (CHCI,) 2974, 2914, 2877, were examined by STM and appeared to have irregularly shaped domains 1209,1049,943,789 cm-I; EIMS m/z (intensity) 131 (M - 1,3) 99 (M that are relatively flat at the nanometer level, separated by nanometer- - SH, 32), 82 (40), 74 (loo), 73 (70), 45 (48). size valleys. 3-(Acetylthio)bicyclo[l.l.l]pentane- I-thiol ([ 115). Yield 72%; bp The gold substrates were either used immediately or were carefully 50-60 “C (50 Torr, sublimator); ‘H NMR 6 2.17 (s, 1 H), 2.26 (s, 3 H), wrapped in lint-free lens paper for storage in a desiccator. The stored 2.35 (s, 6 H); ”C NMR 6 31.02, 37.50, 38.01, 59.95, 195.75; IR (CC4) substrates were used within 48 h and were cleaned by - 1-min immersion 2995,2930, 1702 (C=O), 1205, 1120,941 cm-’; EIMS m/z 131 (M - in hot chromic acid or 1:3 H202/H2S04followed by copious rinsing with Ac, 8), 99 (M -SAC, 9), 58 (17), 45 (13), 43 (100); CIMS 175 (M + water and absolute ethanol and then dried under a jet of dry, filtered 1, 59), 141 (78), 131 (14), 99 (100); CI HRMS m/z (Calcd for C7- argon gas just before use.17 Freshly deposited or cleaned substrates were Hl10S2,175.0251) 175.0236. Anal. Calcd for C7HIOOS2:C, 48.24; H, hydrophilic (H20contact angle < 25O), but they slowly turned hydro- 5.78; S, 36.79. Found: C, 48.27; H, 5.76; S, 36.75. phobic in air (the contact angle after 24 h was -7OO). Both cleaning [2$taffane-3,3’-dithiol ([214).h Yield 72%; mp 126-127 OC (litsa methods afforded identical surface characteristics for the bare substrate 98-105 OC); sub1 70 OC (0.5 Torr); ‘H NMR 6 1.82 (s, 12 H), 1.99 (s, as well as the subsequent modified electrodes. 2 H); “C NMR 6 34.80, 39.98, 55.69; IR 2979, 2906, 2671, 1209, 1147, Fii Preparation. Metal-free staffane self-assembled films were made 892 cm-’; EIMS m/z 165 (M - SH, l), 131 (22), 125 (31), 91 (loo), by immersing freshly deposited or freshly cleaned gold substrates in - 1 65 (21), 59 (51), 45 (59); CIMS m/r 199 (M + 1, 16), 167 (24), 163 mM solutions of [n]4 (film type A) or [n]5 (film type B) in EtOH for (18), 132 (24), 131 (loo), 125 (26), 105 (17). Anal. Calcd for CIOHI4S2: -24 h, rinsed copiously with fresh portions of EtOH, and Ar dried. C, 60.56; H, 7.11; S, 32.33. Found: C, 60.48; H, 7.11; S, 32.23. Films of the ruthenium-derivatized staffanes were prepared by three 3’-(Acetylthio)[Z]staffane-3-thiol([2]5). Yield 40%; mp 74-75 OC; different routes: sub1 (65 OC, 0.25 Torr); ‘H NMR 6 1.85 (s, 6 H), 2.00 (s, 6 H), 2.02 (i) Self-Assembly of Neat Films of [n]6. Mononuclear Rustaffane (s, 1 H), 2.26 (s, 3 H); ”C NMR 6 31.24, 34.86, 37.99, 40.09, 42.80, complexes, [n]6, were dissolved in MeCN to afford - 1 mM solutions, 53.54, 55.74, 196.35; IR 2974, 2908, 2871, 1688 (C=O), 1211, 1119, and the clean gold substrates was introduced to yield a film of type D. 957,894 cm-’; EIMS m/z 207 (M - SH, l), 197, (M - Ac, 4). 165 (16), Self assembly was allowed (-24 h) to reach equilibrium, and the elec- 163 (14), 149 (20), 131 (26), 91 (40), 59 (23), 43 (100); CIMS m/z 241 trodes were washed with MeCN, water, and EtOH. (M + 1,8), 165 (52), 131 (IOO), 77 (45), 65 (75); HRMS m/z (Calcd (ii) In-Situ Synthesis of [n]6. Clean polycrystalline gold substrates for C12H160S2, 240.0643) 240.0646. Anal. Calcd for C12Hi60S2: C, were immersed in 20 pM solutions of [n]5 in absolute EtOH for - 18 59.96; H, 6.71; S, 26.67. Found: C, 60.06; H, 6.72; S, 26.59. h, rinsed copiously with fresh portions of absolute ethanol, and then Ar [3$taffane-3,3”-dithiol ([3]4). Yield 69%; mp 194-195 OC; sub1 100 dried. The resultant electrode was relatively hydrophobic. The dry “C (0.7 Torr); ‘H NMR 6 1.42 (s, 6 H), 1.79 (s, 12 H), 1.97 (s, 2 H); electrode was exposed to 2-5’35 NaOH in EtOH for - 1 min and rinsed ”C NMR 6 34.70, 37.56,40.79,48.38, 55.51; IR 2964, 2904, 2871, 1209, thoroughly with water and absolute EtOH.I8 As shown by FTIR 1190, 1052, 892 cm-’; EIMS m/z 264 (M, 2), 231 (M - SH, 6), 141 measurements, this treatment hydrolyzes the thiol acetate functional (57), 129 (69). 91 (100); CIMS m/z 265 (M + 1,68), 263 (M - 1,68), group to afford a thiol surface that is hydrophilic (film type C). The 231 (M - SH, loo), 229 (74); HRMS m/z (calcd for C1sH20S2 substrate was then exposed to -5 mM aquapentaammineruthenium(I1) 264.1006) 264.1008. Anal. Calcd for CISH2,,S2: C, 68.13; H, 7.62; S, hexafluorophosphate, [Ru(NH,),(H,O)] (PF6)2, in degassed MeCN ov- 24.25. Found: C, 68.14; H, 7.66; S, 24.17. ernight, washed with fresh portions of MeCN, water, and EtOH, and 3”-(Acetylthio)[3)Etaffan~~thiol([315). Yield 38%; mp 125-126 OC; then Ar dried to yield film type E. The sequence of transformations is sub1 105 OC (0.7 Torr); ’H NMR 6 1.45 (s, 6 H), 1.79 (s, 6 H), 1.96 (s, summarized in Figure 1. 6 H), 1.98 (s, 1 H), 2.25 (s, 3 H); I3C NMR 6 31.14, 34.75, 37.65, 37.71, (iii) Coassembly of [n]6 and an Alkanethiol (RSH). Clean polycrys- 38.01, 40.79, 43.65, 48.52, 53.51, 55.64, 196.20; IR 2964, 2905, 2872, talline gold substrates were exposed to a [n]6/RSH (R = CSHllr~CsH17, 1692 (C=O), 1208, 11 14,889,628 cm-l; EIMS m/z 306 (M, 3), 263 or CIOH21)solution (1:lO molar ratio) overnight (total concentration -5 (M - Ac, 4), 231 (M -SAC, 15), 125 (45), 91 (38), 59 (19), 43 (100); mM), to yield film type F. The assembled films were then exposed to HRMS m/z (calcd for C17H220S2,306.1 112) 306.1125. Anal. Calcd a neat 0.1 mM solution of RSH in MeCN for 8 h. This step was carried for C17H220S2:C, 66.63; H, 7.23; S, 20.92. Found: C, 66.51; H, 7.28; out to exchange [n]6 molecules adsorbed at defect sites, grain boundaries, S, 20.83. or film layers adsorbed onto the primary monolayer for RSH and was [4JStaffane-3,3’”-dithiol([414). Used without sublimation; yield 80%; intended to produce films that do not contain electroactive groups in mp 192-193 OC; ‘H NMR 6 1.38 (s, 12 H), 1.79 (s, 12 H), 1.97 (s, 2 metastable environments.19-23 H); ”C NMR 6 34.66, 37.28, 38.19, 40.91, 48.03, 55.50; IR 2964; 2906, 2871, 121 I, 1085; 895 cm-I; EIMS m/z 330 (M, 9), 297 (M - SH, 4), Results and Discussion 129 (39), 125 (73), 115 (36), 105 (40), 91 (loo), 79 (38), 77 (39), 59 (41); HRMS m/z (calcd for C20H26S2,330.1476) 330.1465. In the following, we first describe the preparation of monolayer 3/”-(Acetylthio)[4]taffane-~thiol ([4]5). Used without sublimation; films (A-F) and then the results of the structural characterization yield 23%; mp > 200 OC (decomposition without melting); ‘H NMR 6 of the monolayers A-E by IR spectroscopy and the monolayers 1.39 (s, 6 H), 1.41 (s, 6 H), 1.78 (s, 6 H), 1.96 (s, 6 H), 1.97 (s, 1 H), A-F by contact angle measurements and then present the results 2.24 (s, 3 H); I3C NMR 6 31.26, 34.64, 37.27, 37.39, 37.89, 38.20 (2 of the electrode blocking experiments. We conclude with the C’s), 40.91,43.80,48.04 (6 C’s, two internal cages) 53.31, 55.49, 196.68; results of the electrochemical experiments on the pentaammine- IR 2964,2906,2871, 1694 (C=O), 1213, 1120, 1051,630 cm-l; EIMS ruthenium(I1)-tagged films D and E. The monolayer films were m/z 372 (M, 8), 357 (M - Me, 7), 297 (M -SAC, 14), 157 (32), 143 generally very stable. For example, they could undergo over 100 (31), 129 (46), 128 (38), 125 (93), 105 (34), 91 (49), 43 (100); HRMS m/z (calcd for C22H280S2,372.1582) 372.1579. The purity of the products was checked by GLC. Ru Complexes. Mononuclear ruthenium complexes, [n]6, were pre- (17) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991,63,85. pared from the staffanedithiol [n]3 and separated as hexafluoro- (18) Greene, T. W.; Werts, P. G. M. Protective Groups in Organic Syn- phosphate (PF6-) salts by following published procedures.16 The pres- thesis; J. Wiley & Sons: New York, 1991. (19) Creager, S. E.; Rowe, G. K. Anal. Chim. Acra 1991, 246, 233. ence of the -SRu(NH&*+ ion was verified by cyclic voltammetry at a (20) Creager, S. E.; Collard, D. M.; Fox, M. A. Lnngmuir 1990, 6, 1617. glassy carbon electrode without further characterization. (21) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. Substrate Preparation. For IR and electrochemical studies, 99.999% (22) Sabatini, E.; Moaz, R.; Sagiv, J.; Rubinstein, I. J. Electroanal. Chem. gold was sputtered on glass (borosilicate glass, 2.5 X 7.5 cm2), in a MRC 1989, 219, 365. (Materials Research Corporation, Orangeburg, NY) Model 8620 sput- (23) (a) Kwang, W. S. V.;Atannasoska, L. Miller, L. Lnngmuir 1991, 7, tering system at Torr, with an RF power of 150 W and an RF 1419. (b) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. peak-to-peak voltage of 1.8 kV, using an aluminum mask. The glass Sci. 1988, 200, 45. (c) Chidsey, C. E. D.; Loiacono, D. N. Lnngmuir 1990, 6, 682. (d) Chidsey, C. E. D.; Bertozzi, C. R.; Puvinski, T. M.; Mujsce, A. M. J. Am. Chem. SOC.1990, 112, 4301. (e) Chidsey, C. E. D. Science 1991, (16) Stein, C. A.; Taube, H. J. Am. Chem. SOC.1981, 103, 693. 919. 9946 J. Am. Chem. Soc.. Vol. 114, No. 25, 1992 Obeng et al.
Table I. IR Linear Dichroism of 3,3(R')-Bis(acetylthio)[n]staffanes [n17 n=l n-2 n=3 n=4 VKbVKbV Kb 7 Kb 627 0.39 628 0.38 629 0.42 629 0.47 O.lW1 I I1 I ii R i 0.103 869 0.48 847 931 0.43 895 0.46 892 0.63 892 -0.7 946 0.36 946 0.36 946 0.37 948 0.33 1030 0.28 1051 1050 -0.66 1117 0.38 1115 0.39 1117 0.42 1117 0.49 1140 0.34 1145 0.37 1147 1188 0.49 1190 0.63 1198 -0.6 1204 0.40 1211 0.44 1210 0.64 1213 0.64 1704 0.31 1704 0.28 1702 0.18 1699 0.18 ocm-l. bOrientation factor (see text).
law 14w 1033 6w 18W 14W 1030 6w n = 1-4, as typical, stable in air, and suitable for incorporation 9 (cm") 9 (em") in stretched polyethylene. Figure 2 shows the IR absorption Figure 2. IR linear dichroism of 3,3(*l)-bis(acetylthio)[nlstaffanes, n = spectra of two of the molecules, polarized parallel (E,) and 2 and 3, in stretched polyethylene: Ez (EY),absorbance polarized parallel perpendicular (E,) to the polymer stretching direction. The C-H (perpendicular) to the stretching direction. stretching region was blocked by the absorption of the polymer itself. scans in cyclic voltammetry (100 mV S-I) without change or could The dichroic ratio di = Ez(i)Edi) was converted into the be transferred from one supporting electrolyte solution to another orientation factor Ki = 4/(2 + di) for each observed transition without loss. i in the usual fashionz5(Table I). The physical significance of Preparation of Monolayer Films. Dithiols [a14 and singly these values is Ki = (cosz ai),where atis the angle between the acetylated thiols [a15 were synthesized from the readily available ith transition moment and the stretching direction of the polymer 3,3("1)-bis(acetylthio)[n]staffanes [a]714 by basic hydrolysis. This and the pointed brackets indicate averaging over all observed procedure appears preferable to alternative methods reported5 for molecules. Thus, large Kivalues are characteristic of transitions the preparation of bridgehead bicyclo[l.l .l]pentanethiols. Pu- polarized close to the effective orientation axis of the solute Sication was most easily achieved by elution through a silica gel molecule, and small ones indicate transitions polarized at a large column. The resulting mono- and dithiols were moderately sen- angle to it. The effective orientation axis is that direction in the sitive to air oxidation and were generally dissolved in degassed molecular framework that has the largest ( cosz a) value. ethanol solutions immediately after preparation and stored in a The spectra of all four molecules are very similar, but the freezer. All compounds decompose slowly in the solid state; even orientation factors vary significantly along the series (Table I, in the freezer under argon some yellowing occurred. As evidence Figure 2) and demonstrate an increasing degree of average of physical changes, the melting points become lower, with melting alignment of the effective orientation axis of the solute with the occurring over a broader range with gradual decomposition. stretching direction of the polymer. This was expected, as all prior Elemental analyses remain accurate, however. experience with the methodz5shows that the location of the ef- We have prepared three kinds of metal-free (A, B, C) and three fective orientation axis in the molecule, as well as its degree of kinds of ruthenium-containing (D, E, F) monolayers, obtained alignment, is primarily determined by molecular shape. As the as follows: A, self-assembly from the dithiols, [a14 (n = 2-4); molecules become gradually more rodlikG6 upon going from [ 117 B, self-assembly from the thiol-thiol acetates, [a]!!(n = 2-4, to [4]7, the orientation axis is expected to approach the 3-fold Figure 1); C, hydrolytic removal of surface acetate groups from symmetry axis of the [nlstaffane skeleton and to line up better the monolayer B (n = 2-4, Figure 1); D, self-assembly from the with the polymer-stretching direction. thiol-thiol (pentaammine)ruthenium(II) complexes, [a16 (n = Comparison with the IR spectra of [nlstaffane hydrocarbonsz4 2-4); E, reaction of the monolayer C with aquapentaammine- and consideration of vibrations associated with the thiol acetate ruthenium(I1) (n = 2-4, Figure 1); F, self-assembly from mixtures group identify the intense peak near 1210 cm-' as characteristic of the complexes [a16 and alkanethiol, (n = 2-4). of the [nlstaffane skeleton. The orientation factors listed in Table Attempts to derivatize the dithiol monolayers A with aqua- I leave no doubt that this vibration is long-axis polarized. In the pentaammineruthenium(I1) did not yield reproducible results. following, we shall use the intensity of this parallel vibration and IR Spectra. (i) Transition Moment Directions from Mea- those of the C-H stretching vibrations at 2850-3000 an-', which s"ts on [ap. To interpret the IR spectra of the films in terms must clearly be of perpendicular polarization in view of the of molecular orientation, a few characteristic and intense vibrations orientation of the CH2 groups within the [nlstaffane skeleton, to of the [nlstaffane skeleton need to be identified and their po- determine the orientation of [nlstaffane rods in thin films. larization determined. Given the 3-fold symmetry of a rigid (ii) Grazing Incidence IR Spectra of Monolayer Films. The [nlstaffane hydrocarbon,2" the allowed IR vibrations will be po- results for both staffane lengths, n = 2 and 3, are identical, and larized either along the long axis or in the plane perpendicular it seems safe to assume that the same results would be obtained to it, and the latter will be degenerate. The terminal substituents for the longer staffanes. IR absorption spectra of the immobilized present in our molecules will lower the overall symmetry, but the monolayers on a reflecting metal surface are distorted relative transition moment directions of the vibrations of the skeleton itself to solution spectra in ways that are fairly well understood,z7and will be dictated by the high local symmetry, so that the such distortions are also manifested in our spectra. In particular, "parallel"-"perpendicular" nomenclature will apply. the monolayer absorption peaks, primarily the broader ones, are Incorporation in stretched polyethylene represents a well-es- shifted to somewhat higher frequencies, and in our spectra, this tablished means of aligning nonpolar molecules for dichroic is most clearly apparent for the carbonyl stretch. Also, in some spectr~scopy.~~We have chosen the diacetylated dithiols [a]7, cases, reflection is actually weakly enhanced, so that the absorption peaks appear with a negative amplitude. In our spectra, this is (24) (a) Murthy, G. S.; Hassenrock, K.;Lynch, V. M.; Michl, J. J. Am. barely detectable in the CH stretching region and not seen Chem. Soc. 1989, Ill, 7262. (b) Gudipati, M. S.; Hamrock, S. J.; Balaji, V.; Michl, J. J. Phys. Chem., in press. (25) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light. (26) Friedli, A. C.;Lynch, V. M.; Kaszynski, P.; Michl, J. Acto Crys- Solute Alignment by Photoselection in Liquid Crystals, Polymers, and tallogr., B 1990, 46, 317. Membranes; VCH Publishers: Deerfield Beach, Florida, 1986. (27) Allara, D.;Baca, A,; Pryde, C. A. Macromolecules 1978, 11, 1215. /n/Staffane-3,3(”’)- Dithiols J. Am. Chem. Soc., Vol. 114, No. 25, 1992 9941
A
A n=3 10.001 n =3 T
1250 1150
Figure 4. Grazing incidence IR absorption spectra of monolayers of types B (solid line) and C (dash-dotted line) on a gold substrate: dotted line, 2900 00 isotropic spectrum of the thiol acetate [0]5 in a KBr pellet, plotted on i7 (cm-l) a scale such as to make the intensity of the 1210-cm-’ peak qual to that Fwe3. Grazing incidence IR absorption spectra of monolayers of type inthe film of type B. A on a gold substrate: dotted line, isotropic spectrum in a KBr pellet, plotted on a scale such as to make the intensity of the 1210-cm-I peak Within our experimental accuracy, the CH stretching bands qual to that in the film. either are absent altogether or are present as weak negative peaks elsewhere. These anomalies, and the relatively low signal-to-noise in the spectra of the monolayers self-assembled from the thiol-thiol ratio, call for caution in any quantitative interpretation of the acetates [a]S, whether before hydrolysis (type B) or after hy- results, and we suspect that the orientation of molecular axes drolysis (type C). The observed spectra represent an extreme case cannot be determined with an accuracy better than &lo0. of the situation encountered with monolayers of type A, and in In a molecule located next to a conductor surface, only that the first approximation, the molecules in the films of types B and component of a transition dipole that is perpendicular to the C are standing with their long ax= normal to the surface. There surface contributes to light absorption to the first approximation, is no detectable difference between the alignment of the [nlstaffane due to the effect of image charges in the conductor. If the or- cores in the films of types B and C, suggesting that the hydrolytic ganization of the molecules near the surface is not isotropic, those step does not perturb the structure of the monolayer. The transitions whose polarization directions in the molecular frame somewhat reduced absorption intensity of the 1210-cm-l band in tend to lie parallel to the surface will have relatively reduced the spectrum of films C relative to films B is reproducible, and intensity. In our case, these are the staffane CH2 stretching we suspect that it is due to the perturbations of the IR transition vibrations, and we conclude that, on the average, the long axes moment by the electric fields due to the acetyl substituent charge of the molecules in monolayer A form a small angle with the distribution and its mirror image in the metal. There is no reason normal; i.e., the molecules are “standing up” on the surface. We to think that it is due to partial destruction of the monolayer and cannot distinguish from these measurements between a situation removal of some of the staffane rods upon going from film B to in which some of them are standing up absolutely straight while C. As we shall see below, film C conserves excellent blocking others are lying down and a situation in which all of them are properties, and upon going to film E, the intensity of the 1210-cm-’ standing up but not quite straight. To obtain quantitative in- band increases again, even above that of film B. formation on molecular orientation in the films from the relative The integrated intensity of the carbonyl stretch band in film intensities of the 2850-3000 and 1210-cm-’ vibrations, the gold B relative to that of the 1210-cm-’ peak is smaller by a factor surface is assumed to be perfectly flat. of about 0.62 for n = 2 and 0.41 for n = 3 when compared with (M)Monolayers of Type A. Figure 3 shows selected regions the spectra of the isotropic samples. This result contains infor- of the grazing incidence of the IR absorption spectra of monolayers mation on the average conformation of the acetylthio group in of type A on a gold substrate for n = 2 and 3. The ordinary the monolayer. A priori, one would expect it to be either cis planar, isotropic IR absorption spectra recorded in KBr pellets are shown with the CSCO dihedral angle r#J equal to zero (the 0 atom as well. The integrated intensities of the CH stretches relative pointing to the metal surface and the CH3 group outward), or to those of the 1210-cm-I band are smaller in the monolayer trans planar, r#J = 180°, (the 0 atom pointing outward and the spectra by a factor of about 0.37 in the case of n = 2 and a factor CH3 group toward the metal). of about 0.49 in the case of n = 3. Allowing for the degeneracy Relative to that of the 1210-cm-’ peak, which is polarized along of the short-axis polarized transitions, we obtain an average of the molecular long axis and thus normal to the surface, the in- 45O and 40° for n = 3 and 2, respectively, for the angle between tensity of the carbonyl stretching vibration is reduced in the film the [nlstaffane long axis and the surface normal. It is clear that, spectrum by a factor of (cos2 a). The pointed brackets indicate in the films of type A, the average inclination of the [nlstaffane ensemble averaging and u is the angle between the surface normal rods is about halfway between normal and parallel to the surface. and the C=O bond. (iv) Monolayers of Types B and C. Figure 4 shows the spectra Some of the variation in the intensity ratio observed between of fhof types B and C. In the spectra of the former, the central n = 2 and 3 may be due to the effects just discussed for the region contains a c-0 stretching vibration near 1700 an-’.The 1210-cm-’ band, and some may reflect real conformational dif- polarization direction of the carbonyl stretch can be expected to ferences. In view of the uncertainties in the interpretation of lie close to the C4bond direction, but its orientation relative intensity differences, we prefer not to attempt an analysis of minor to the molecular axis will be a function of the molecular con- effects. We assume that the C-0 bonds in all the thioacetyl formation. The C-H stretching intensity due to the methyl group groups in the film make the same angle u with the surface normal is difficult to separate reliably from that due to the [nlstaffane and take lcos 61 to be equal to 0.7 f 0.1. CHI groups but is likely to be relatively small, particularly for Two ranges of CSCO dihedral angles are compatible with the n = 3. observations. They are particularly simple to derive if the staffane 9948 J. Am. Chem. SOC.,Vol. 114, No. 25, 1992 Obeng et al. rods are assumed to be exactly perpendicular to the surface and if the bridgehead C-S bond is oriented exactly along the long axis of the molecule. Then, the angle w between the S-CO bond and I0.0007 the surface normal is equal to the CSC valence angle. If the SCO valence angle is labeled p, trigonometry yields cos 4 = (cos w cos p - cos a)/sin w sin p The use of the average values of the valence angles obtained from single-crystal X-ray structures of the bis(acety1thio) derivatives [2]7, [3]7, and [4]7,26w = 102.5O and p = 123.0°, yields two solutions: (i) 4 = 0-29O and (ii) 4 = 126-146'. In the singlecrystal structures, the bridgehead C-S bond is bent backward, away from the acetyl group, and in some cases, the staffane rod itself is slightly bent in the same direction as well. This distortion is presumably due to the need to accommodate the neighboring molecules. If a similar need exists in the packed monolayer at all, it is likely to be less severe. If the resulting tilt of the bridgehead C-S bond from the surface normal away from the acetyl group is T, the proper value to use for w in the calculation of the dihedral angle 4 becomes the sum of the CSC valence angle and the tilt angle T. The tilt reduces the magnitude of permitted values of 4 somewhat. For instance, for a tilt of 7 = 5O, the permitted ranges of 4 are (i) 0-17' and (ii) 123-143O. Two ranges of the dihedral angle 4 characterizing the acetylthio Figure 5. Grazing incidence IR absorption spectra of monolayers of types group are compatible with observations: (i) cis, with 4 possibly D (dotted line) and E (solid line) on a gold substrate. deviating from zero by perhaps as much as 30° (carbonyl oxygen pointing inward), and (ii) approximately gauche, with 4 about coverage by ruthenium pentaammine, the long-axis polarized 12C-150°. The planar or nearly planar conformation (i) is normal 1210-cm-'peak characteristic of the [nlstaffane rod is weaker for the acetylthio group, is similar to the single-crystal structures in the film of type D by a factor of at least 2. Since the rods are of the bis(acety1thio) derivatives,26 and is quite plausible. The oriented equally in both films, the large intensity difference is best twisted conformation (ii), although compatible with the IR in- attributed to a difference in their numbers. tensity ratios, appears highly improbable. The trans conformation This actually is to be expected and is consistent with the (carbonyl oxygen pointing outward) is excluded. electrochemical surface coverage measurement discussed below. The striking difference in the average orientation of the [n]- The radius of the staffane rod in the monolayer is probably close staffane rods in the films of types A and B is most readily ra- to 2.9 A, judging by the crystal structures of a series of substituted tionalized by postulating that the favored mode of attachment staffanes26 and by the 26 A2 per molecule area in the Lang- of the thiol sulfur to the gold surface is with the C-S bond normal muir-Blodgett monolayer of ~taffane-3-carboxylates,*~and that to the surface, presumably pointing into the center of a triangle of the roughly spherical ruthenium pentaammine unit is closer formed by three gold atoms, and that this mode of attachment, to 3.5 Since the C-S bond is perpendicular to the surface, characteristic of films B and C, is also used by about half of the the packing of the staffanes is probably commensurate with that staffanedithiol molecules in the films of type A. An attachment of the gold lattice with the S located at a 3-fold site. In the absence with the C-S bond parallel to the surface requires a sulfur-to- of Ru(NH~)~~+substitution, we would expect a closer packing surface distance of about 2.7-3.0 A, imposed by the thickness of that accommodates the molecular area. However, in the ru- the [nlstaffane rod:6 and is apparently only about half as fa- thenation step in which type E films are produced, the densest vorable, since in films A the formation of two such bonds, one commensurate packing is 2(d3 X d3) and at least half of the on each rod end, seems to compete about evenly with the favored staffane rods must remain metal-free. The actual packing by mode of attachment. Although we have no experimental evidence Ru(NH~)~~+may well be less dense because of electrostatic re- for such bimodal orientation distribution in films A in the absence pulsions among these positively charged centers. of Raman spectra, it appears more reasonable than other possi- By the same token, self-assembly from [o]6 cannot pack the bilities compatible with the observed average angle. It is difficult ruthenium pentaammine termini any closer than 2(d3 X d3), to see why a different angle of inclination would be favored by and the packing density of the staffane rods underneath is at best the gold-attached thiol group depending on the presence or absence half that found in films of type E. It is not certain how the voids of an acetyl protecting group on the distant terminus of the rod, in film D are filled, but a likely possibility is the counterions. There if that other terminus did not interact with the gold surface. is no evidence for water in the IR spectra of dried films. (v) Monolayers of Types D and E. Figure 5 shows the grazing As we shall see below, electrochemical measurements provide incidence IR spectra of monolayers of types D and E. As would independent evidence that the coverage with Ru atoms is the same be expected, these are dominated by bands due to the five ammonia in films D and E, supporting the conclusion drawn here from the molecules present on each ruthenium atom: the broad N-H approximate equality of the ammonia IR bands in the two spectra. stretching band between 3000 and 3400 cm-I, the degenerate The existence of such independent evidence is reassuring in view deformation band near 1600 cm-l, and the symmetric deformation of the disquieting variation of the 1210 cm-I band intensity be- band at 1333 cm-1.28 The Ru-S stretching vibration lies outside tween films B and C. Comparison of Figures 4 and 5 shows that our observation range, and we have no information on the C-S-Ru the conversion of film C into E is associated with an increase in angle. The three C-H stretching bands of the [nlstaffane rods the 1210 cm-l band intensity, even above that of film B. However, are again absent within our detection ability, demonstrating that, we find it unlikely that the reproducible difference in the intensity in films D and E, the rods are still standing perpendicular to the of this peak in films D and E could be wholly due to the same surface. effect. Still, we are sufficiently concerned about these intensity There is one striking and reproducible difference between the variations that we hesitate to interpret the ratio of the two as two spectra shown in Figure 5, however. While the ammonia representing quantitatively the ratio of the staffane rod densities peaks have a comparable intensity in both, suggesting equal surface
(29) Yang, H. C.; Magnera, T. F.; Lee, C.; Bard, A. J.; Michl, J. Lung- (28) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Co- muir, In press. ordination Compounds; Wiley: New York, 1986. (30) Meyer, T.J. Acc. Chem. Res. 1978, 11, 94. /n]Staff~ne-3,3(~')-Dithiols J. Am. Chem. SOC..Vol. 114, No. 25, 1992 9949
Table 11. Contact Angle of Water on the Monolayer Modified Electrodes' n typeb If ed 8. 6, AI 62f2 66f2 52f2 2 85f2 69f2 87f2 3 76f2 69f2 80f2 4 81f2 69f2 86k2 B2 78f2 67f2 80f2 3 74f2 61f2 84f2 4 74f2 66f2 84f2 C 1 26f2 20f2 27f2 2 38 f 10 34f 2 38 f 10 3 43f9 39f2 43f5 D2 63f2 61f2 66f2 3 63f2 51f2 65f2 E2 42f6 36f6 44f6 3 53f4 51k1 60f1 F I/CSHIISH 38 f 11 29f 11 44f 11 2/CSHIISH 67 f 7 62 * 5 69 f 8 67 f 9 82 f 12 3/CBHI7SH 78 f 11 I I I, t 'A sessile 10-pL drop of Milli-Q water. bFilm types: A = [n]4, 0.8 dithiol; B = [n]S, thiol-thiol acetate; C = [n]4, dithiol, D,E = [n]6, 0.6 0.2 .0.2 ruthenium complex; F = [n]6 and RSH, mixed film. 'Number of re- E I Volts vs SCE peat units in rod. dThe stationary contact angles (e, deg) were mea- Figure 6. Cyclic voltammogram of a 1.2 mM solution of K,Fe(CN),. sured with stage normal, while the advancing (&., deg) and receding 3H20at a bare gold electrode (solid line), a gold electrode with a mon- (er, deg) contact angles were measured with the sample inclined at 45'. olayer of 1415 (type B, dashed line), and a gold electrode with a mono- layer of [4]4 (type A, dotted line). Scan rate 50 mV s-[, SCE reference on the surface in the films D and E. electrode. (vi) Contact Angle Measurements. The results of contact angle measurements obtained with deionized water (pH = 5.5) on the 1.200 t I various films support the structural assignments based on IR (Table 11). The values for the thiol-thiol acetates [n]5,type B, 1.ooo are high, as would be expected for a surface covered with methyl groups. They are in relatively good agreement with the angle (70°) 0.800 reported for the aliphatic thiol-thiol acetate HS(CH2)12SCOC- e H3.IzbThe dithiol films of type A show even higher contact angles than the films of type B, in keeping with the notion that about half of the staffane rods lie flat on the surface, exposing much pure hydrocarbon surface to the contacting solution. The thiol-terminated films of type C (derived from the thiol acetate) 0.200' ' * . " ' . " ' . ' . have low contact angles, again not surprisingly. -0.5 0.0 0.5 1.0 1.5 2.0 2:5 3.0' The contact angles obtained on Ru-containing films of types Log t D and E were essentially the same within experimental error but depended on the length of the staffane rods. The small differences between films D and E are not understood. Both films were more 0.01 0 I hydrophilic than films A and B, and in contrast to the purely organic films, both the pure and mixed films containing staffanes . derivatized with ruthenium showed higher uncertainty in the 0.008 C observed contact angle, indicating rather heterogeneous surfaces.lZb - The surface heterogeneity can be explained in terms of irregu- 0.006 larities in the distribution of the counterions and the Ru(NH~)~~+ moieties over the film. The apparent dependence of the contact angles on the staffane rod length suggests that the contact angle is sensitive to the underlying substrate.j' (Vu) BIocking properti@ of the Fii. The cyclic voltammetric current-potential (i-E) responses for a bare gold electrode (solid 0 line) in a 1.2 mM solution of Fe(CN),+ with 0.1 M KCl are shown Log t in Figure 6. The corresponding cyclic voltammograms for an Figure 7. Plot of the current transient at an electrode with a monolayer electrode with a blocking monolayer of a film of type A ([4]4) of (314 (film type A) and of [3]5 (films of type B and C, respectively), and an electrode with a blocking monolayer of film type B (1415) normalized to the current transient at a bare gold electrode vs log (time) are also shown. A strong decrease in both the oxidation and in a 10 mM solution of Ru(NH3),CI3 (0.1 M H2S04). reduction waves is apparent for the type B monolayer. The monolayer of type A does not block the solution redox reaction 0.5 M H2S0, for films of types A, B, and C, each for n = 1-4, nearly as efficiently; the oxidation and reduction wave decreased as well as with film types D and E (n = 2, 3). These were then by -70%. The same finding was made for all A, B pairs (at all analyzed according to the pinhole model proposed by Matsuda chain lengths). Voltammograms obtained with films of type C et al.I5 In this model, the chronoamperometric behavior of the were identical to those of type B. diffusing reactants is described at a partially coated electrode; In an attempt to obtain a quantitative measure of the area of the amount of the electrode left exposed is obtained from the short the electrode that remained exposed after the monolayer film had time limit of a plot of the current at a modified electrode divided been prepared, current transients were recorded both at the bare by the semi-infinite diffusion current obtained at a bare elec- and blocked electrodes in a 10 mM solution of Ru(NH3),C13 in tr~de.~~~.~Figure 7 shows such a plot for electrodes with blocking monolayers of types A, B, and C. The short time results suggest (31) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A,; Porter, M. that the exposed area is about 30%of the total electrode area for C. J. Am. Chem. SOC.1991, 113, 2370. film A but less than 1% for the films B and C. The blocking 9950 J. Am. Chem. SOC.,Vol. 114, No. 25, 1992 Obeng et al.
I I I I I I 0.7 0.6 0.5 0.4 0.3 0.2 E (VOLT)
b
4 1.2 0.8 0.4 ov
E Volts V6 Ag ORE Figure 8. Gold surface oxidation and reduction waves (a) at a bare gold electrode, (b) with a monolayer of [3]4 (film type A), and (c) with a monolayer of [3]5 (film type E) in -0.1 M H2S04,pH 1.6, v = 50 mV d.8 01.7 d.6 0',5 O!S OI.3 d.2 0I.l O'.O S-'. E (VOLT) efficacy of the monolayer films decreases with decreasing staffane Figure 9. Cyclic voltammogram of (a) [2]6 (film type E) on a poly- length for n I2. For the films B and C, the blocking is -95% crystalline gold substrate (geometric area = 0.18 cm2) and (b) a 2 mM for n = 2 and -99% for the longer staffanes. For n = 1, blocking solution of [2]6 with a glassy carbon electrode, in 1 M HCIO,, v = 200 films were not obtained. The apparent blocking efficiency by the mV s-', with a SCE reference electrode. Ru-containing films was lower: -70% for type E (n = 2,3) and Ru(H,O)(NH,)~(PF,),. If the chemisorbed staffane rods were for type (n = 2). The low efficiency found with the type -55% D all tilted regularly at -45O away from normal, one of the thiol E film compared to the much better efficiency for the precursor groups would be available for functionalization. type C film suggests that the coordinated Ru(NH&~+groups in (viii) Electrochemistry of Films of Types D and E. In Figure the type E film may be partially mediating electron transfer to 9 the electrochemical properties of a neat film of [2]6, adsorbed the solution probe species and the actual coverage and direct from 1 mM solution in MeCN (type D), are compared with those blocking may be higher. of a 2 mM solution of [2]6 at a glassy carbon electrode in 1 M An alternative method for the determination of blocking ef- HC104. Results obtained with a film of type E obtained by ficiency, based upon the decrease in the gold oxide reduction wave, metalation of film C were identical and are not shown. A glassy has also been used as a direct measure of the degree of blocking carbon electrode was used in the solution experiments because in self-assembled monolayers.22 Figure 8a shows the oxidation the complex adsorbs on noble metal electrodes. The peak currents and reduction waves obtained at a bare gold electrode in H2S04 for both the oxidation and reduction of the immobilized [2]6varied = at pH 1.6, Figure 8b shows the corresponding cyclic voltam- linearly with scan rate over scan rates of 25 mV s-I to 50 V s-l. mogram for a gold electrode covered with a self-assembled film The peak potential (E ) of the immobilized material of 0.51 V of type A, and Figure 8c shows the cyclic voltammogram obtained vs SCE, at v = 200 me s-l, is about 60 mV more positive than with a monolayer of type B or C, which gave identical results. that of the solution species. This suggests that the Ru(I1) in the If the potential was scanned positive of + 1.7 V vs a Ag QRE, the film is stabilized with respect to Ru(III), which may be attributed monolayer was destroyed, presumably due to oxidation of the to an increased electrostatic interaction between the Ru-centers gold-bound sulfur, and the film no longer blocked the electrode. in the +3 oxidation state. Alternatively, the shift in peak potential The results for each of the compounds calculated using the three of the film may be a consequence of the differences in specific methods are in relatively good agreement. In general, the films ion pairs formed between the electroactive group and the ClO; of type A were found to only block 60-80% of the gold oxide ions from the supporting ele~trolyte.'~The charge passed under reduction peak and solution redox reaction, compared to the >95% the anodic wave, after background correction, was - 12 pC cm-2 blocking of the gold oxide reduction and solution redox reaction (geometric area of electrode) or 1.2 x 1@Io mol cm-2. The densest by films of types B and C. The less effective blocking of the commensurate packing of the Ru-staffane molecules would be electrode surface by the films of type A is consistent with the FTIR 2(d3 X d3)R30°, which affords 83 A2 per molecule projected structural evidence discussed above. If about half of the molecules area per RU(NH,)~~+center or 2.0 X 1O-Io mol cm-2 per mono- are present with both thiol groups chemisorbed on the gold surface, layer. The roughness of the polycrystalline Au substrate used was in a more or less random fashion, and about half are standing 1.6 f 0.2, as determined by the charge passed for the reduction upright, the rods are likely to be disorganized, with the probability of an adsorbed iodine monolayer on bare Au.~~Thus, the observed of a greater number of pinholes relative to that for the films of type B and C. That the film type A may not be coherent is further indicated (32) (a) Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P.; Villegas, 1.; Suggs, D. W.; Stichey, J. L. J. Phys. Chem. 1991, 95, 5245. (b) Rodriguez, by the fact that redox waves for the Ru2+I3+couple were not J. F.; Soriaga, M. P. J. Electrochem. Soc. 1988, 135,616. (c) Rodriguez, S. reproducibly obtained when the type A films were derivatized with P.; Mebrahtu, T.; Soriaga, M. P. J. Electroanul. Chem. 1987, 233, 283. /n/Staffane- 3,3('4-Dit hiols J. Am. Chem. SOC.,Vol. 114, No. 25, 1992 9951
I *i m -1.0 0.0 1.o 2.0 3.0 PH Figure 11. Variation of the oxidation peak potential (E,) for both dis- solved and surface immobilized [2]6 (film types D and E) in 1 M HClO,. Y = 200 mV S-I.
I I I I I I 0.7 0.6 0.5 0.4 0.3 0.2
E (VOLT) Figure 10. Cyclic voltammogram of (216 (film type E) on a polycrys- talline gold (area = 0.18 cm2) in (a) 0.5 M H2S04, (b) 2 M CF3COOH, and (c) 0.1 M HCI. Y = 200 mV s-l. I I I 1 i coverage is equivalent to about 0.4 monolayer of Ru(NH~)~~+, 0.7 0.6 0.5 014 0.3 0.2 which is similar to that observed by Finklea and Hanshew for HS(CH~),(C-~)-NH
Intramolecular Magnetic Coupling between Two Nitrene or Two Nitroxide Units through 1,l-Diphenylethylene Chromophores. Isomeric Dinitrenes and Dinitroxides Related in Connectivity to Trimethylenemethane, Tetramethyleneethane, and Pentamethylenepropane
Takuya Matsumoto, Takayuki Ishida, Noboru Koga, and Hiizu Iwamura* Contribution from the Department of Chemistry, Faculty of Science, The University of Tokyo, 7-3-1 Hongo. Tokyo 113, Japan. Received April 27, 1992
Abstract: Isomeric vinylidenebis(pheny1nitrenes) (N)and 1 ,l-bis[(N-oxy-tert-butylamino)phenyl]-2-methylpropenes(0) have been prepared, and their EPR fine structures and/or effective magnetic moments have been determined over a wide temperature range. The data were analyzed in terms of Bleaney-Bowers-type equations to give the energy gaps between the quintet and singlet states for N and the triplet and singlet states for 0. They were >>0 cm-I, -42.0 cm-’ (-502 J/mol, a negative value representing a singlet ground state), and -26.2 cm-’ (-313 J/mol), and 10.6 cm-’ (127 J/mol), -1.8 cm-l (-22 J/mol), and -3.4 cm-’ (-41 J/mol) for p,p’-, m,p’- and m,m’-dinitrenes N and dinitroxides 0,respectively. A consistent reduction in the effective intramolecular exchange integrals Ss between the two open-shell centers in the latter series is ascribed to the lower spin polarization on the phenyl rings of the dinitroxides relative to the dinitrenes. Whereas the ferro- and antiferromagnetic couplings in the pp‘ and m,p’-topology are consistent not only with the Ovchinnikov’s but also Borden-Davidson’s rules, the antiferromagnetic interaction in the m,m’-substitution pattern is contradictory to the former and consistent with the latter theory. Since the p,p’, m,p’, and m,m‘ isomers are related to trimethylenemethane (4), tetramethyleneethane (S), and pentamethylenepropane (6), respectively, the results obtained in this work are consistent with the triplet ground state of 4. 5 and 6 are suggested to have ground singlet states. Radical centers X should be all at the para position for designing super-high-spin polyacetylenes 1 and polydiacetylenes 2.
As a part of our efforts to design and construct new organic on such polymers so far reported are unsatisfactory or only partly polymers in high-spin ground states,’ we became interested in successful2in contrast to well-characterized high-spin ground states knowinn if uolv(uhenv1acetvlenes) 1 and uolv(uhenvldiacetv1ene-s) 2 carry:& ;r& Yadicil centers X‘as pendanidbn tie phenil ring$ (2) Fujii, A.; Ishida, T.; Koga, N.; Iwamura, H. Macromolecules 1991, be high-spin and how strong the ’pins in 24, 1077. Inoue, K.; Koga, N.; Iwamura, H. J. Am. Chem. SOC.1991, 113, couple each other in the polymer nKdecules. Experimental results 9803. Nishide, H.;Yoshioka, N.; Inagaki, K.; Tsuchida, E. Macromolecules 1988, 21, 31 19. Nishide, H.;Yoshioia, N.; Kaneko, T.; Tsuchida, E. Mac- romolecules 1990, 23, 4487. Yoshioka, N.; Nishide, H.; Tsuchida, E. Mol. (1) Iwamura, H. Adu. Phys. Org. Chem. 1990, 26, 179. Cryst. Liq. Cryst. 1990, 190, 45.
0002-7863/92/15 14-9952%03.00/0 0 1992 American Chemical Society